In a conventional cannon, a projectile is accelerated by the rapid expansion of gases resulting from the explosive combustion of propellant chemicals. The muzzle velocity of a projectile shot from a cannon is generally only slightly greater than the initial acoustic velocity of the expanding gases. This limitation results because the ballistic efficiency of the chemical propellant charge decreases rapidly as the driving gas expends most of its energy in accelerating itself. Thus, the decreasing ballistic efficiency of an expanding propellant charge inherently limits the acceleration of a projectile through the bore of a conventional cannon.
To overcome the limitation on projectile velocity imposed by driver gasdynamics, a new method for accelerating projectiles has been developed that does not use an exploding propellant charge, but instead, continuously burns a combustible gas mixture to accelerate a projectile in a method referred to as "ram acceleration". This method is based on principles similar to those used in the air breathing ramjet engine, but is substantially different in many respects. For example, a ramjet engine carries with it a supply of fuel; in comparison, the projectile in a ram accelerator does not carry any propellant. Instead, the projectile travels through a tube filled with a mixture of gaseous combustible fuel and an oxidizer compressed to several atmospheres of pressure. The tube functions like the outer cowling of a ramjet, and the profile of the projectile has a shape much like the central body of a ramjet. As the projectile passes through the combustible gas mixture, the gaseous mixture flows past the "throat," i.e., the largest diameter portion of the projectile body, into a diffusion area disposed immediately behind the throat and burns in a combustion zone proximate the aft portion of the projectile. Combustion of the gaseous fuel process in a forward moving combustion zone, producing an increased pressure that accelerates the projectile down the bore of the tube. The ballistic efficiency of the ram acceleration process may be maintained at a high level by tailoring the combustible gas mixture in the tube to maintain the projectile Mach number within prescribed limits.
At least five modes of ram acceleration are theoretically possible in the ram accelerator, depending upon the profile of the projectile, its velocity, and other operational factors. In one of the modes, referred to as a "thermally choked mode," combustion of the gas mixture proceeds at subsonic velocities behind the projectile, accelerating the projectile to velocities in the range of from 0.7 to 3.0 kilometers per second. The thermally choked mode can be used to initially accelerate the projectile once the ram acceleration process is started. Then, by transitioning the projectile to one of the other modes, it can be accelerated to even higher velocities. Muzzle velocities as high as 12 kilometers per second may thus be achieved.
Early problems with operating a laboratory test prototype ram accelerator in the thermally choked mode and the solutions to these problems are described in U.S. patent application Ser. No. 207,706, filed June 16, 1988, now U.S. Pat. No. 4,982,647. In that invention, as has typically been true of all ram accelerators, the projectile is preaccelerated to a supersonic velocity before it enters a portion of the tube filled with the combustible gas mixture. A shock wave caused as the projectile enters the combustible gas mixture is throttle to insure that its velocity is less than or equal to that of the projectile, thereby establishing a subsonic flow past the projectile to initiate a stable combustion zone proximate the aft end of the projectile.
The preferred method previously used for preaccelerating the projectile to supersonic velocities before it enters the combustible gas mixture employs a tank of compressed helium. The projectile is placed in a portion of the tube that has been evacuated, and a fast-acting valve is opened, allowing the compressed helium to expand into the evacuated portion of the tube behind the projectile. A sabot or disk that is slightly smaller in diameter than the bore of the tube is positioned immediately behind the projectile. The expanding helium forces the sabot and projectile to accelerate down the tube to a supersonic velocity. As the moving projectile perforates a membrane separating the evacuated portion of the tube from a first section that is filled with the combustible gas, it initiates thermally choked ram acceleration. To throttle the resulting shock wave sufficiently to provide a stable subsonic combustion zone behind the projectile, a perforated or relatively lightweight sabot is used. Alternatively, a port can be provided in the tube wall proximate where the projectile enters the portion of the tube filled with the combustible gas mixture, or other techniques can be employed to throttle the shock wave, as described in the above-referenced patent application.
The prior art teaches that a chemical propellant, e.g., an explosive charge, can also be used for preaccelerating a projectile to a supersonic velocity to initiate the ram acceleration process. To use a chemical propellant, the projectile is typically loaded into a breech capable of withstanding the pressure created by the explosive ignition of the chemical propellant and is fired into the first segment of the tube filled with combustible gas, just like an artillery shell. This technique for preaccelerating a projectile has its drawbacks, however. Ignition of the chemical propellant is likely to produce a substantial recoil. The weight of the breech and requirements for handling the recoil clearly impact on options for placement and mounting of the ram accelerator.
The above-described techniques for preaccelerating a projectile to initiate a ram acceleration process add to the complexity, size, weight, and logistical considerations involved in operating the ram accelerator. Accordingly, it is an object of the present invention to initiate ram acceleration of a projectile without preaccelerating it. It is further an object to "start" the ram acceleration process using an expanding combustible gas mixture. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follow.